SWTC Question (and FuzzFace scenario)

[Derringer]

here's Mark's write-up
http://hammer.ampage.org/files/SWTC.gif

In his design, the resistor R sets the highest treble roll-off point.


I'm tinkering with a FuzzFace design right now and I'm wondering if there is anything intrinsically wrong with disregarding the R resistor of the SWTC in certain scenarios.
Case-in-point, I have the tone pot DC coupled to Q2's collector. I am reasoning that I do not need an additional R resistor between the SWTC and the collector because the resistance looking into the collector itself sets the highest treble roll off. Does my reasoning seem straight?


2ndly ... and more specifically to the design I'm working on, in order to try and consolidate some parts, I am using a 25KB pot as both the SWTC and the Collector resistor. (I'm able to use that large of a collector resistor because my feedback resistor is only 39K.) So one lug is connected to +9V, the other to the Q2 Collector and the wiper to a 0.01 cap to GND (connected to V+ works too). And then I also have a 0.1 cap connected to that same Q2 Collector, then to another resistor (to drop the volume a bit) and then to the output pot.

The only design I've seen before where a pot doubles as a collector resistor is in the Rangemaster.
Does this seem like sound designing?
It sounds great like this on my breadboard.

Thanks

[Mark Hammer]

The SWTC is essentially a "decontextualized tone control" in the sense that it makes no presumptions about what comes before it, only about what comes after it.  It's like a pickle on the side of a sandwich plate; doesn't really matter what's in the sandwich.

If you are in any position to be able to calculate out what the rolloff might be for a given cap, with the wiper moved all the way over to the "input" side of the pot, then more power to ya.  What I showed makes no such presumptions and does the calculating based ion the small-value resistor in series with the input lug...simply because that's what I know how to do.  If the principle works in a predictable manner in the total absence of an input resistor, cool.  Just let me know how you worked it out.

[Derringer]

and in looking at your design, that's pretty much what I had figured ... no presumptions.  Like if the SWTC is set directly after a very low Z opamp output, then having a limiting resistor would be nice so that you "could" set a high freq. point. I think that I can visualize a bit of a problem occuring if the input Z is really low compared to the tone pot because I'd imagine the high freq points might get a bit bunched up because there would be such an abrupt change from say 2K down to something in the 100's of ohms? (I really don't know ... can only speculate ... but why would you have a LPF there anyway if it's always going to be set to the highest degree.)


How did I work this cobble of parts out? Mostly study, trial, introduce error, measurement, adjust, repeat.
No real math is involved, just a rudimentary understanding of what I'll just call "magnitudes of change."

thanks

[stringsthings]

here's Mark's write-up
http://hammer.ampage.org/files/SWTC.gif

In his design, the resistor R sets the highest treble roll-off point....



... and more specifically to the design I'm working on, in order to try and consolidate some parts, I am using a 25KB pot as both the SWTC and the Collector resistor. (I'm able to use that large of a collector resistor because my feedback resistor is only 39K.) So one lug is connected to +9V, the other to the Q2 Collector and the wiper to a 0.01 cap to GND (connected to V+ works too). And then I also have a 0.1 cap connected to that same Q2 Collector, then to another resistor (to drop the volume a bit) and then to the output pot.

The only design I've seen before where a pot doubles as a collector resistor is in the Rangemaster.
Does this seem like sound designing?
It sounds great like this on my breadboard.

your logic appears sound to me ( no pun intended ) ... the definition of design is somewhat slippery ... if the circuit sounds good to you, then it is good - regardless of how the result was achieved ...

i like Mark Hammer's SWTC because of its simplicity and flexibility ... ( you can tailor the response to your specific app ... some folks like one pickle on the side and some folks like two 

oh yeah, and it has a cool acronym 

[Derringer]

oh yeah, and it has a cool acronym 

+1

thanks for your input

[Mark Hammer]

I would not call it a "design".  It's just.....a thing.  And its not mine.  It's something I saw somewhere in passing.

As for the acronym, I always get a warm feeling seeing it because it is so close to SWTP, who were DIY pioneers in the 70's.